Ultrasonic imaging is used as a diagnostic tool to aid in therapeutic procedures. It is based on the principle that waves of sound energy can be focused upon an area of interest and reflected to produce an image. Generally, an ultrasonic transducer is placed on a body surface overlying the area to be imaged, and ultrasonic energy, produced by generating and receiving sound waves is transmitted. The ultrasonic energy is reflected back to the transducer where it is translated into an ultrasonic image. The amount and characteristics of the reflected energy depend upon the acoustic properties of the tissues, and contrast agents which are echogenic are preferentially used to create ultrasonic energy in an area of interest and improve the image received.
In ultrasound imaging, videotape images obtained following contrast injection are digitized, allowing the gray scale to be quantified from 1 to 225 gray scale units for 30 cardiac cycles. The contrast intensity is plotted on the vertical axis against time on the horizontal axis. The peak videointensity (corrected for baseline intensity) is determined as the highest point on the time intensity curve.
For a discussion of contrast echographic instrumentation, see, for example, De Jong N, "Acoustic properties of ultrasound contrast agents", CIP-GEGEVENS KONINKLIJKE BIBLIOTHEEK, DEN HAG (1993), pages 120 et seq.
Contrast echocardiography has been used to delineate intracardiac structures, assess valvular competence, and demonstrate intracardiac shunts. Myocardial contrast echocardiography (MCE) has been used to measure coronary blood flow reserve in humans. MCE has been found to be a safe and useful technique for evaluating relative changes in myocardial perfusion and delineating areas at risk.
A multiplicity of potential ultrasonic imaging agents has been reported for contrast echocardiography. No such agent routinely attains visually discernible myocardial uptake following peripheral intravenous injection. Although there have been many reports of transpulmonary transmission of ultrasound contrast agents following intravenous injection and despite the fact that myocardial opacification on echocardiogram can be produced by left sided injection of such contrast agents, visualization of myocardial contrast has not been achieved by intravenous administration of sonicated microbubbles.
Most recently, sonicated albumin and sonicated dextrose/albumin have been shown to produce variable degrees of left ventricular chamber ultrasound contrast following intravenous injection. (See Villanueva et al. Circulation 85:1557-1564, 1992; Lin et al. Int J Card Imaging 8:53-6, 1992; Feinstein et al. J Am Coll Cardiol 16:316-224, 1990; Keller et al. Am Heart J 114:570-575, 1987; and Shapiro et al. J Am Coll Cardiol 16:1603-1607, 1990). The microbubbles of these contrast agents are small (4-6 microns) and are capable of swift transpulmonary passage. However, visually discernible myocardial uptake of such microbubbles following peripheral intravenous injection has not been possible because of the rapid diffusion of blood soluble oxygen and nitrogen inside the microbubble into the blood which consequently loses its ultrasound reflective properties (e.g., see Porter et al. J Am Soc Echocard Supplement 7:S1, May 1994, and Weyman AE: Principles and Practice of Echocardiography, Malvern, Pennsylvania: Lea & Febiger, 1994; pp. 302-26.)
Despite recent advances in contrast agents comprising injectable gas encapsulated microbubbles, several problems remain for adequate detection and visualization of the organ of interest. The bubbles are plagued with filtration by capillaries, diffusion of gas to the liquid medium, and lack of concentration at the organ of interest due to dilution.
Attempts to solve these problems have led to studies of acoustic velocity of media containing gas bubbles, second harmonic emission, and resonance frequency. These studies to date have met with little improvement in contrast visualization.
It is an object of the present invention to provide a safe, simple and effective method to visualize and improve an ultrasound image following injection of gas-filled microbubbles.
Other objects of the invention will be apparent from the description of the invention which follows.